SEERiously, What’s Behind a Rating?

Written by Bruce Manclark, Contextual Analyst

SEER (Seasonal Energy Efficiency Ratio) is used to gauge an air conditioner’s efficiency and to market higher efficiency equipment to consumers. For the record, SEER is the ratio of BTUs removed divided by watt hours drawn. It does include some other factors, such as the coefficient of degradation (cycling losses). The test is at 82°F outside, and inside conditions at 80°F dry bulb and 67°F wet bulb; in other words, a condition that we who are lucky enough to live West of the Rockies seldom see. We have dry summers and when we air condition a house for a few hours, the air becomes even drier. (The test also includes fan wattage, but it’s at an unrealistic ESP that causes the fan energy to be artificially low.)

The following is from an ACEE Proceeding that describes the SEER rating in detail:

A key concept for SEER is the “bin” method. Temperature bins are just the frequency distribution of cooling hours for a season in a given location. For constant speed systems, the calculation procedures were simplified to eliminate the need to use temperature bin data. Instead the procedure simply uses performance data from the single outdoor temperature of 82°F, which happens to be the load-weighted average of the temperature bin data.

The rating procedure still compensates for part-load degradation by assuming the air conditioner is sized to be 50% loaded at 82°F. The calculations for single-speed air conditioners become very simple:

SEER = (1 – Cd*0.5) *EER82

Manufacturers can choose to forgo cyclic laboratory testing and use a default value of 0.25 for Cd. However, since most modern systems have a Cd of 0.10–0.15 (Dougherty 2003; CEC 2006), most manufacturers choose to determine Cd by testing their equipment. This simple calculation procedure was adopted instead of a temperature bin method in 1979 because it yielded good results with fewer calculations. The bin method is required for equipment with multiple compressors or with compressors that can vary their output with load. Thus, the standard for two-stage equipment requires steady-state measurements at 82°F and at 95°F at each operating stage.

A potential problem with this simplification was that it caused manufacturers to focus design efforts on maximizing EER (energy efficiency ratio) at 82°F, since the EER at 95°F is not included in the simplified calculations (only capacity matters at 95°F).

Comparisons between wet bulb and dry bulb temperatures indicate the amount of moisture in a sample of air. The closer they are together (the wet bulb depression), the more moisture is in the air; the further they are apart, the drier. On a hot day, it not unusually in the West to have indoor wet bulb tempers of below 60°F.

George Box is quoted as saying “that which gets measured tends to improve.” True, but if you are measuring the wrong thing, the outcome might not be positive. Perhaps an update might be “that which gets measured and marketed tends to create confusion.”

Proctor Engineering and others have suggested a sensible EER at a given outdoor condition, usually 95°F. EER and SEER both use the total capacity of the system (latent and sensible) in their calculations. When the West gets hot, we are dry, so we really don’t get care what the latent capacity. The sensible EER is easily calculated by dividing the sensible capacity at design condition by watt hours. Unfortunately, this means digging beyond AHRI and looking at the detailed capacity charts.

The chart below is a two typical 2-ton A/C unit. It’s rated at SEER 14. Test conditions are at 95°F and 800 cfm. The detailed capacity tables give only the wattage of the outdoor unit, so we added indoor fan wattage using the default fan wattage outlined in the SEER procedure. Please notice that the total EER increases as the wet bulb increases and the sensible EER decreases.

So, what does it all mean?

SEER is not a good indicator of efficiency for our dry summer climate. In fact, many high SEER pieces of equipment have the same sensible EER as SEER 14 equipment. Regardless of the equipment chosen, the most critical detail is the quality of installation. Low air flow and incorrect charge will reduce any equipment’s ratings to that of a 20-year-old air conditioner. In the West, where we enjoy our dry summers, it is best practice to choose an air conditioner that maximizes the sensible capacity at 95°F.